Three-phase controllers use a principle of phase gating to control the amount of energy supplied to an electrical load. This allows the starting currents and torques in asynchronous machines to be reduced. The phase gating angle is used as a measure of the amount of energy supplied. Current half-cycles of alternate polarity flow through the load which is connected to the output of the three-phase controller with there being a time interval during which no current flows and which is determined by the phase gating—also referred to as the delay—between each two successive current half-cycles.
Three-phase controllers are normally equipped with three pairs of thyristors which are connected back-to-back in parallel. However, since the thyristors become the cost-determining factor as the rating of the three-phase controller increases, three-phase controllers having only two pairs of thyristors which are connected back-to-back in parallel are also used. In these so-called two-phase three-phase controllers, the remaining third phase is in the form of a conductor which cannot be switched. A drive such as this is known from DE 30 09 445 A1.